the white house has just announced that president obama will travel to boston for an interstate service on thursday. laura joins us from a city still in shock. >> yes, cathy, a very somber city. as you just heard in the update from fbi officials in boston, someone knows who did this. they said they are appealing to the public for clues. we also have an update on the forensic examination of the site where the bomb went off. it seems the bomb was contained in a pressure cooker and carried in a black nylon backpack. at the moment, no more questions have been answered. my colleague has this report. >> runners were struggling to the finish line, their families watching, at cheering. then the unthinkable. cheers became screams. a moment of celebration and triumphed shattered. on the streets of boston, sights and sounds more familiar in countries at war. pavement spattered in blood as two bomb blasts claimed their victims. it was the cruelest of moments. >> i saw a trash can explode. people started throwing down the barricade and running over each other. i ran in the other direction as fast as